The preferred raw material for production of melamine is urea. Ammonia and carbon dioxide are by-products in the reaction which may be either high pressure and non-catalytic, or low pressure and catalytic using a catalyst such as alumina. The basic reaction is ##STR1## The temperature of the reaction, depending on conditions, will vary but is usually between about 350.degree. and 400.degree. C. (662.degree. to 752.degree. F.). The by-products, ammonia and carbon dioxide, are commonly returned to an adjacent urea plant from which the starting material, a urea melt, is obtained for the melamine reaction. Melamine product is recovered by either a water quench and recrystallization, or by sequential cooling and filtering of the effluent gas from the reaction. The melamine product customarily is at least 99% pure.
Four commercial processes typify melamine manufacture from urea, i.e., the BASF, Chemie Linz, Nissan, and Stamicarbon procedures. All of the presently practiced commercial processes require substantial energy in the form of steam, electricity, and natural gas. The total energy consumed in these operations varies from 11,000 BTU/lb. melamine product to 23,000 BUT/lb. melamine product. The energy consumed in the reaction of urea to produce melamine is approximately 2200 BTU/lb. The remainder of the energy consumed in the commercial processes is a result of the complexity of the processes and the equipment utilized, and primarily as a result of the separation of the offgas from the product and purification of the product which normally includes a water quench and recrystallization, or a fractional condensation of the melamine and impurities.
In the BASF process melamine is manufactured by heating urea to temperatures of from 350.degree. to 450.degree. C. at atmospheric or low pressures, i.e., up to about 10 atmospheres, in the presence of catalysts and added ammonia. The reactor, built to contain catalyst as well as urea at pressures only slightly above atmospheric, is relatively large. BASF, U.S. Pat. Nos. 4,138,560 and 3,513,167, believed to be directed to the BASF process, describe the melamine as being separated from the reaction gases by fractional condensation, filtration, and cooling the gases to temperatures of from 150.degree. to 250.degree. C. Unreacted urea is removed by further cooling. The by-product ammonia is removed as an offgas from the reactor containing carbon dioxide at slightly above atmospheric pressure. The offgases transferred to the urea synthesis plant at atmospheric pressure is necessarily compressed before use in urea synthesis. It is difficult and costly to bring the offgases to the high reaction pressure required for urea conversion in large-scale production because carbamate can condense if the compression is carried out at relatively low temperature, causing a corrosion problem; and the volume of gases handled can be very large if the compression is carried out at relatively high temperature. The use of alumina catalyst in the BASF process can create problems associated with formation of lumps. Elaborate thermocouple systems in the reactor interior are necessary to forewarn operators of impending hot spots, and the reactors must be shut-down to permit steam feed to remove such lumps. Catalyst escaping from the reactor is removed from the product gases by the use of filters. Heating coils in the reactor are corroded by the severe conditions. The BASF process consumes about 12,000 BTU/lb. melamine formed.
The Chemie Linz process is a two-stage, low-pressure catalytic system. In the first stage urea is decomposed in a fluidized sand bed. Melamine is produced in a second-stage fixed alumina catalyst bed. The melamine product is recovered by quenching the hot reaction gas with cooling aqueous liquor and centrifuging the resulting slurry. Ammonia and carbon dioxide and recovered in two separate streams readily usable for different processes. Ammonia gas is recovered from the offgas at about atmospheric pressure. Carbon dioxide is produced at about 300 psig (20 atms). The Chemie Linz process consumes about 14,500 BUT/lb. melamine product formed.
According to the November 1970 issue of Hydrocarbon Processing, the Nissan Chemical process takes place at 100 kg/cm.sup.2 (94.5 atms) and 400.degree. C. (752.degree. F.) in the absence of a catalyst. Melamine product from the reactor is cooled in a pressure quencher into aqueous ammonia solution. This solution, after separating part of the ammonia at medium pressure, is filtered and reduced to atmospheric pressure in a recrystallizer where the remaining ammonia is separated out and melamine is crystallized out. Melamine crystals separated from the crystallized melamine slurry are centrifuged, dried, and pulverized into the final product. Use of high pressure permits a reduction in the size of the reactor; however, because the mixture is corrosive, the smaller reactors must be made of titanium alloys or other alloys which are non-corrosive. Water is needed to prepare the aqueous ammonia solution used in quenching the reactor product stream, and is needed to wash the melamine crystals in the recrystallization process. According to U.S. Pat. No. 3,454,571, assigned to Nissan Chemical Company, believed to be directed to the Nissan process, an aqueous alkaline solution wash is required to remove impurities adhering to the melamine crystal surface in order to obtain high-grade melamine. The Nissan process consumes about 11,000 BTU/lb. melamine product.
The Stamicarbon melamine process is a low-pressure catalytic system in which melamine is precipitated from the hot reaction gas by quenching rapidly with an aqueous mother liquor. The melamine is purified by dissolving, blending with activated carbon, filtration, and recrystallization. The water is removed by passing the recrystallized product through hydrocyclones, centrifuges, and a pneumatic dryer. After completing these drying steps, the crystalline product is collected. The offgas is produced as a concentrated carbamate solution at 212.degree. F. (100.degree. C.) and 265 psig (18 atms), and returned to the urea synthesis stream. Recycling the carbamate solution to the urea plant introduces additional water to the urea process, reducing conversion to urea. The catalyst in this process must be kept fluidized, and can become agglomerated if cold spots appear causing lumping or condensation of the catalyst. The use of an alumina catalyst requires that makeup catalyst be supplied to the reactor to replace catalyst fines contained in the reaction gas. The Stamicarbon process consumes about 23,000 BTU/lb. melamine product formed.
As is apparent, each of the aforesaid processes suffers disadvantages from a practical standpont. In the low-pressure system where the melamine goes directly to a vapor without passing through a liquid melamine stage there are few impurities. However, the low-pressure reactor and recovery system are complex, requiring extensive equipment and space, and consume high amounts of energy including as a result of the handling of large volumes of gases. Additionally, since a catalyst is employed, separate problems are presented in the separation or filtration of the product from the catalyst. In the known high-pressure systems where the melamine is first formed as a liquid, substantial amounts of impurities are normally found in the melamine product including significant amounts of melam and melem which are detrimental for the end uses of the melamine product. Accordingly, in the known high-pressure systems it has been necessary to utilize an aqueous quench, recrystallization, and subsequent drying of the melamine product to obtain the necessary degree of purity, requiring complex and space-consuming equipment, as well as high energy consumption.